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This question is an extension for my previously asked question here: Why is the voltage across collector and emitter pushing itself to 0.7 volts?

A colleague of mine suggested that I take a look at the stability factor for the circuit in that question. I calculated it and found it to be 284, which is very high and bad, and I thought that this is what was driving the transistor into saturation. But then re-did my calculations by assuming a lower stability factor of 10. From this, I got new values of resistors as follows:

R1 = 4.7 K ohms

R2 = 1.2 K ohms

Rc = 2.2 K ohms

Re = 100 ohms

After making these changes, when I tested the DC conditions by applying a supply voltage of 5 volts, there were no changes in the results. The Vce voltage still remains at 0.7 volts, i.e almost in saturation, contrast to my expected value of 2.5 volts. Here are the various voltage drops I get:

Across R1 : 4.6 volts

Across R2: 0.4 volts

Across Rc: 4.2 volts

Across Re: 0.2 volts

I have even tried changing the transistor. The transistor I used is 2N3904C NPN. Please enlighten me and provide me with a solution on how to overcome this.

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  • \$\begingroup\$ Your measurements don't make sense. Your schematic in your old question specifies a 9V supply, but your R1 + R2 voltages only add up to 5V. \$\endgroup\$ – brhans Feb 4 '16 at 16:16
  • \$\begingroup\$ Hmm, in my 30 years experience with circuits like these (you might want to include the schematic BTW) I've never heard of a stability factor for such a simple circuit. Can you explain what you mean by stability factor ? Before you can design such a circuit you need to understand functions and relations of all the components. I would design such a circuit on a piece of paper. Only a few hand calculations are needed here and there. Then put it in the simulator to confirm that it does what I think it should do. \$\endgroup\$ – Bimpelrekkie Feb 4 '16 at 16:18
  • \$\begingroup\$ Perhaps you have your base and collector leads swapped. This could explain the consistent 0.7 volts Vce, which if swapped would be Vbe \$\endgroup\$ – Marla Feb 4 '16 at 16:25
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On this question, and on your previous question, you keep getting 0.7 volts Vce.

Perhaps you have swapped the base and collector leads. This would put the base emitter leads across your "emitter" resistor, resulting in 0.7 volts no matter how you bias the resistors R1 and R2.

2N3904C

enter image description here

EDIT : If you have connected transistor properly, you can place a jumper across your R2 (short it out, making it zero ohms), and Vce should rise dramatically. Do not leave the jumper installed after this test.

After satisfying that your circuit is wired properly, increase value of R1 (perhaps to 6.8K) and increase in small steps until you see Vce begin to rise in voltage.

I have added a schematic to represent what the OP has in his previous question, and with his current values.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ i've connected the leads the right way. i've checked many times. \$\endgroup\$ – Sâu Feb 4 '16 at 16:39
  • \$\begingroup\$ Then put a jumper across R2. Vce has to rise dramatically. \$\endgroup\$ – Marla Feb 4 '16 at 16:51
  • \$\begingroup\$ i don't understand. what do you mean by jumper? \$\endgroup\$ – Sâu Feb 4 '16 at 17:09
  • \$\begingroup\$ Place a wire across R2, making R2 zero ( 0 ) ohms. Do this just for a test to see if transistor is connected properly. Do not leave R2 at zero ohms after the test. \$\endgroup\$ – Marla Feb 4 '16 at 17:12
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Your main problem is that, following the answer to your previous question, you made the RC/RE ratio too high. Consider what is going on:

1) R1/R2 set the base voltage at ~1 volt.

2) This sets the emitter voltage at ~0.3 volts (give or take a bit).

3) So the emitter current should be ~3 mA.

4) The collector current should be about the same as the emitter current, so the collector resistor should have a voltage of (3ma x 2.2k) or about 6.6 volts.

5) 0.3 plus 6.6 equals 6.9 volts, which is noticeably larger than your 5 volt supply. This explains why your transistor is in saturation.

6) So replace Re with. let's say, a 330 ohm resistor. Then the current will be about 1 mA, and the collector resistor should drop about 2 volts. You should wind up with about 2 to 2.5 volts across the transistor, and have a signal gain in the range of about 7.

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  • \$\begingroup\$ Alright, I checked this now. Here is the result: Voltage drop across collector is 3.5 volts and across emitter is 0.8 volts. And hence Vce is still at 0.7 volts. \$\endgroup\$ – Sâu Feb 5 '16 at 2:26
  • \$\begingroup\$ Disconnect the base and leave it floating. Now try the circuit. If you still get 0.7 volts you've connected the transistor wrong, per Marla's answer. If that's not the problem, try a different transistor. \$\endgroup\$ – WhatRoughBeast Feb 5 '16 at 3:13
  • \$\begingroup\$ transistor is connected correctly. I've also changed several transistors. The voltage still remains at 0.7 volts \$\endgroup\$ – Sâu Feb 5 '16 at 10:02
  • \$\begingroup\$ Then your circuit does not correspond to Marla's schematic. Either a) you have miswired your circuit, or b) you have one or more components whose value differs from what you have listed. You have a DMM. Measure each of your resistors and verify their values. When you disconnected the transistor base, what voltages did you get? Collector, emitter, and bias resistor junction. \$\endgroup\$ – WhatRoughBeast Feb 5 '16 at 15:00
  • \$\begingroup\$ The connections are correct. Components values are also same. And Vce is still at 0.7 volts. \$\endgroup\$ – Sâu Feb 5 '16 at 15:30

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